1. Field of the Invention
The present invention relates generally to analog/digital recording and playback systems. More particularly, the invention relates to optical systems for retrieving data from a storage medium such as an optical tape.
2. State of the Art
Known optical memory systems record information by directing a scanning beam of laser light to produce spots representing bits of data on a moving storage medium, such as a flexible optical tape. For example, FIG. 1 shows five parallel tracks of data spots 36 recorded on a moving optical tape 38. The data spots 36 have a reflectivity, transmissivity or other optical characteristic that can be distinguished from the background of the optical tape 38.
Typically, optical reading systems use a narrow beam that reads a single track of data spots. For example, in the FIG. 1 embodiment, a detector 40 reads data stored on the optical tape 38 and includes three segmented photodetectors 42, 44 and 46 for detecting reflected light. The photodetectors 42, 44 and 46 are used in connection with a three beam light source and are arranged to receive light reflected from selected tracks numbered 1, 3 and 5 respectively.
The three beam light source reads a single track of stored data by illuminating the optical tape with three relatively narrow, focused beams 30. Each of the narrow beams 30 is associated with a given one of the photodetectors 42, 44 or 46, respectively. The detector 44 is typically used for data read-out while the detectors 42 and 46 are used for tracking.
As referenced herein, the term "narrow" refers to a light beam which illuminates an area on a recording medium that does not exceed the width of one recorded track of data spots. Because narrow beams 30 are used to read data, each of the photodetectors 42, 44 and 46 has a relatively wide area for detecting reflections from the optical tape.
The limited ability of optical systems to simultaneously read more than one track of flexible optical tape is largely due to the use of conventional tracking techniques. For example, FIG. 2a of the present specification corresponds to U.S. Pat. No. 4,730,293 and shows a servo controlled tracking technique for reading and writing a rigid storage medium such as an optical disk or data card.
For a rigid optical disk having a fixed center of rotation, data stored on the disk is recorded in a single line which continues around the disk. Using mechanical servo control techniques, the information is typically recorded in data tracks which rotate at an approximate speed of 20-30 Hz. A mechanical arm supporting the laser head is servo controlled to accurately align the laser beam with the data tracks.
Referring to FIG. 2a, spots representing bits of data can be written onto a rigid disk 37 by directing a laser light beam 14 from a laser light source 13 toward the disk through a collimating lens 21, a beam splitter 25, and a quarter wave plate 41. The light beam is focused on the disk through a final optical lens 32.
Data is retrieved (i.e., read) from the disk by scanning a beam 17 from a lower intensity laser light source 23 onto the disk. The light beam 17 is directed toward the disk through a lens 27, the beam splitter 25 and the lens 32. The light beam 17 is deflected, scattered or transmitted by the previously recorded spots to one or more photodetectors.
As described at column 3, line 52 of the aforementioned patent, lens 32 may be centered with respect to an optical axis 35. In this case, light deflected by the disk is directed along an optical path 44 to a detector 29 by lens 32, quarter wave plate 41, a beam splitter 45, and a lens 43. U.S. Pat. No. 4,730,293 further discloses the use of a differential output from two detectors (e.g., detectors 104 and 106 in FIG. 6 of the patent) as a servo control signal for mechanically aligning the laser light source with a recorded data track.
Although conventional tracking systems as described above are relatively accurate when reading data from a rigid storage medium, these same systems are extremely susceptible to alignment errors when reading data from a flexible storage medium. For example, the side to side movement of a flexible optical tape presents increased potential for misalignment between the light source, the moving optical tape and the detector. This misalignment can cause inaccurate tracking and reading of the recorded data, especially where a scanning light source is used. Further inaccuracies due to tracking can significantly increase when the data is not accurately recorded on the optical tape.
Where difficulties in tracking arise, additional error checking and correcting measures are typically proposed. However, error checking often involves reading recorded data more than once, thereby reducing the overall system speed. Where multiple laser beams are used for tracking, further reductions in reading speed may be necessitated due to the increased potential for misalignment between the laser heads and the plural detectors.
Accordingly, there is a need for a system capable of reading data from an optical storage medium, such as an optical tape, which is free of the aforementioned drawbacks (e.g., misalignment errors) so that high speed data acquisition and performance can be achieved.